The present invention relates generally to the packaging of semiconductors, and more specifically to attaching dies to lead frames.
U.S. Pat. No. 5,286,679 discloses a method for patterning polyimide on a surface of a semiconductor wafer in a number of discrete regions. The polyimide is used as an adhesive for attaching a die to a lead frame. The regions of polyimide are patterned to avoid covering saw streets where the wafer will be sawed to make up the individual dies. After the wafer is sawed, the individual dies can be mounted on a lead frame using one of a number of known processes, such as a lead-on-chip (LOC) or a lead-under-chip (LUC) process.
If the patterning of polyimide on the wafer is not performed accurately, it can cause problems at the saw; moreover, visual recognition of the die attach will note a defect if polyimide is bridging the saw street and/or the bond pad area. Defects in the adhesive pattern also cause poor die attachment. Polyimide is very difficult to remove without also damaging circuitry formed in the wafer. Consequently, a wafer would typical be discarded rather than reworked. To avoid the waste of having to discard the wafer, it would be desirable to have a method for removing such polyimide without damaging the substrate.
Similar problems can arise after a die is bonded to a lead frame. In some cases, due to a defective lead frame or due to poor patterning of the polyimide, the die attachment may be ineffective. It would be desirable to be able to rework the die and the lead frame to save them rather than requiring that they be discarded.